In current 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems, Releases 8, 9 and 10, downlink (DL) control signaling from a base station (or eNB) is received by a User Equipment (UE) in the first 1/2/3/4 symbols of a sub-frame. The remaining symbols are used for receiving data. Control signaling is spread across the entire carrier bandwidth (BW) of the sub-frame and the control signaling is received by the UE on a Physical Downlink Control Channel (PDCCH). Data is received by the UE in select Resource Blocks (RBs) occupying either the entire carrier BW or a portion of the BW. Data is received on Physical Downlink Shared Channel (PDSCH). The frame structure received at the UE is illustrated in FIGS. 1A through 1C.
The UE needs to perform channel estimation after receiving the PDCCH to decode the information sent on PDCCH. To perform channel estimation, UE receives Reference Signals (RSs) or pilot symbols in the sub-frame. The reference symbols are associated with one or more antenna ports. For LTE Releases 8, 9 and 10, the UE uses the reference signals associated with one or more of antenna ports 0, 1, 2, 3 for receiving the PDCCH. The RS structure for antenna ports 0, 1, 2, 3 is shown in FIGS. 1A through 1C wherein resource elements R0, R1, R2, R3 carry reference signals associated with antenna ports 0, 1, 2, 3, respectively. An antenna port is defined such that a channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed.
For LTE Release 10 (Rel-10), for demodulating data (sent on PDSCH) the UE can either use reference signals associated with antenna ports 0, 1, 2, 3 or use reference signals associated with all or a subset of other antenna ports 7, 8, 9, 10, 11, 12, 13, 14 based on the transmission scheme used for PDSCH reception. In 3GPP LTE, the transmission scheme depends on configuration signaling from eNB. The reference signals associated with these other antenna ports are typically referred to as “UE specific reference signals (UERS)” or “Demodulation reference signals (DMRS)” or “Dedicated reference signals (DRS)”. The reference signals associated with antenna ports 0, 1, 2, 3 are typically referred to as “Common Reference Signals (CRS)”. While the CRS are sent across the entire carrier bandwidth by the eNB, DMRS can only be present in those RBs for which the UE has a PDSCH assignment. So, for receiving PDSCH using DMRS, the UE can only use the DMRS present on those RBs for which it has a PDSCH assignment.
For LTE Rel-11, it is envisioned that new DL control signaling will be sent by the base station to the UE in symbols that span a first slot of the sub-frame or in symbols that span both the first and second slots of the sub-frame. The new DL control signaling is generally referred to as the Enhanced—PDCCH (E-PDCCH). Unlike the PDCCH, which is transmitted across the entire channel bandwidth, the UE is expected to receive the E-PDCCH in a set of RBs that may span only a portion of the carrier bandwidth in the frequency domain. Also, unlike the PDCCH which is received by the UE using CRS, it is envisioned that the E-PDCCH can be received by the UE using DMRS.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.